Regulation of hematopoiesis and the hematopoietic stem cell niche by Wnt signaling pathways

Hematopoietic stem cells （HSCs） are a rare population of cells that are responsible for life-long generation of blood cells of all lineages. In order to maintain their numbers, HSCs must establish a balance between the opposing cell fates of self-renewal （in which the ability to function as HSCs is retained） and initiation of hematopoietic differentiation. Multiple signaling pathways have been implicated in the regulation of HSC cell fate. One such set of pathways are those activated by the Wnt family of ligands. Wnt signaling pathways play a crucial role during embryogenesis and deregulation of these pathways has been implicated in the formation of solid tumors. Wnt signaling also plays a role in the regulation of stem cells from multiple tissues, such as embryonic, epidermal, and intestinal stem cells. However, the function of Wnt signaling in HSC biology is still controversial. In this review, we will discuss the basic characteristics of the adult HSC and its regulatory microenvironment, the ＂niche＂, focusing on the regulation of the HSC and its niche by the Wnt signaling pathways.     

Luteinizing hormone signaling restricts hematopoietic stem cell expansion during puberty

The number and self‐renewal capacity of hematopoietic stem cells (HSCs) are tightly regulated at different developmental stages. Many pathways have been implicated in regulating HSC development in cell autonomous manners; however, it remains unclear how HSCs sense and integrate developmental cues. In this study, we identified an extrinsic mechanism by which HSC number and functions are regulated during mouse puberty. We found that the HSC number in postnatal bone marrow reached homeostasis at 4 weeks after birth. Luteinizing hormone, but not downstream sex hormones, was involved in regulating HSC homeostasis during this period. Expression of luteinizing hormone receptor (Lhcgr) is highly restricted in HSCs and multipotent progenitor cells in the hematopoietic hierarchy. When Lhcgr was deleted, HSCs continued to expand even after 4 weeks after birth, leading to abnormally elevated hematopoiesis and leukocytosis. In a murine acute myeloid leukemia model, leukemia development was significantly accelerated upon Lhcgr deletion. Together, our work reveals an extrinsic counting mechanism that restricts HSC expansion during development and is physiologically important for maintaining normal hematopoiesis and inhibiting leukemogenesis.     

Hematopoietic stem cells: generation and self-renewal

Adult stem cells hold great promise for future therapeutic applications. Hematopoietic stem cells (HSCs) are among the best-characterized adult stem cells. As such, these cells provide a conceptual framework for the study of adult stem cells from other organs. Here, we review the current knowledge of HSC generation during embryonic development and HSC maintenance in the bone marrow (BM) during adult life. Recent scientific progress has demonstrated that the development of HSCs involves many anatomical sites in the embryo, but the relative contribution of each of these sites to the adult HSC pool remains controversial. Specialized anatomical sites in the BM have been identified as stem cell niches, and these play essential roles in regulating the self-renewal and differentiation of HSCs through recently identified signaling pathways. Extracellular signaling from stem cell niches must integrate with the intracellular molecular machinery and/or genetic programs to regulate HSC fate choice. The exact cellular and/or molecular mechanisms defining stem cell niche and 'stemness' of HSC is largely unknown although substantial progress has been made recently. Hence, many questions remain to be answered even in this relatively well-defined model of stem cell biology.     

In vivo divisional tracking of hematopoietic stem cells

Hematopoietic stem cell (HSC) division leads to self-renewal, differentiation, or death of HSCs, and adequate balance of this process results in sustained, lifelong, high-throughput hematopoiesis. Despite their contribution to hematopoietic cell production, the majority of cells within the HSC population are quiescent at any given time. Recent studies have tackled the questions of how often HSCs divide, how divisional history relates to repopulating potential, and how many HSCs contribute to hematopoiesis. Here, we summarize these recent findings on HSC turnover from different experimental systems and discuss hypothetical models for HSC cycling and maintenance in steady-state and upon hematopoietic challenge.     

Hematopoietic stem cell development and regulatory signaling in zebrafish

Background

Hematopoietic stem cells (HSCs) are a population of multipotent cells that can self-renew and differentiate into all blood lineages. HSC development must be tightly controlled from cell fate determination to self-maintenance during adulthood. This involves a panel of important developmental signaling pathways and other factors which act synergistically within the HSC population and/or in the HSC niche. Genetically conserved processes of HSC development plus many other developmental advantages make the zebrafish an ideal model organism to elucidate the regulatory mechanisms underlying HSC programming.

Scope of review

This review summarizes recent progress on zebrafish HSCs with particular focus on how developmental signaling controls hemogenic endothelium-derived HSC development. We also describe the interaction of different signaling pathways during these processes.

Major conclusions

The hematopoietic stem cell system is a paradigm for stem cell studies. Use of the zebrafish model to study signaling regulation of HSCs in vivo has resulted in a great deal of information concerning HSC biology in vertebrates.

General significance

These new findings facilitate a better understanding of molecular mechanisms of HSC programming, and will provide possible new strategies for the treatment of HSC-related hematological diseases, such as leukemia. This article is part of a Special Issue entitled Biochemistry of Stem Cells.     

Functional compensation between hematopoietic stem cell clones in vivo

In most organ systems, regeneration is a coordinated effort that involves many stem cells, but little is known about whether and how individual stem cells compensate for the differentiation deficiencies of other stem cells. Functional compensation is critically important during disease progression and treatment. Here, we show how individual hematopoietic stem cell (HSC) clones heterogeneously compensate for the lymphopoietic deficiencies of other HSCs in a mouse. This compensation rescues the overall blood supply and influences blood cell types outside of the deficient lineages in distinct patterns. We find that highly differentiating HSC clones expand their cell numbers at specific differentiation stages to compensate for the deficiencies of other HSCs. Some of these clones continue to expand after transplantation into secondary recipients. In addition, lymphopoietic compensation involves gene expression changes in HSCs that are characterized by increased lymphoid priming, decreased myeloid priming, and HSC self‐renewal. Our data illustrate how HSC clones coordinate to maintain the overall blood supply. Exploiting the innate compensation capacity of stem cell networks may improve the prognosis and treatment of many diseases.     

Thrombopoietin and hematopoietic stem cells

Thrombopoietin (TPO) is the cytokine that is chiefly responsible for megakaryocyte production but increasingly attention has turned to its role in maintaining hematopoietic stem cells (HSCs). HSCs are required to initiate the production of all mature hematopoietic cells, but this differentiation needs to be balanced against self-renewal and quiescence to maintain the stem cell pool throughout life. TPO has been shown to support HSC quiescence during adult hematopoiesis, with the loss of TPO signaling associated with bone marrow failure and thrombocytopenia. Recent studies have shown that constitutive activation mutations in Mpl contribute to myeloproliferative disease. In this review, we will discuss TPO signaling pathways, regulation of TPO levels and the role of TPO in normal hematopoiesis and during myeloproliferative disease.     

Temporal Changes in PTEN and mTORC2 Regulation of Hematopoietic Stem Cell Self-Renewal and Leukemia Suppression

Pten deletion from adult mouse hematopoietic cells activates the PI3-kinase pathway, inducing hematopoietic stem cell (HSC) proliferation, HSC depletion, and leukemogenesis. Pten is also mutated in human leukemias, but rarely in early childhood leukemias. We hypothesized that this reflects developmental changes in PI3-kinase pathway regulation. Here we show that Rictor deletion prevents leukemogenesis and HSC depletion after Pten deletion in adult mice, implicating mTORC2 activation in these processes. However, Rictor deletion had little effect on the function of normal HSCs. Moreover, Pten deletion from neonatal HSCs did not activate the PI3-kinase pathway or promote HSC proliferation, HSC depletion, or leukemogenesis. Pten is therefore required in adult, but not neonatal, HSCs to negatively regulate mTORC2 signaling. This demonstrates that some critical tumor suppressor mechanisms in adult cells are not required by neonatal cells. Developmental changes in key signaling pathways therefore confer temporal changes upon stem cell self-renewal and tumor suppressor mechanisms.     

Oxidative stress-mediated mitochondrial fission promotes hepatic stellate cell activation via stimulating oxidative phosphorylation

Previous studies have demonstrated dysregulated mitochondrial dynamics in fibrotic livers and hepatocytes. Little is currently known about how mitochondrial dynamics are involved, nor is it clear how mitochondrial dynamics participate in hepatic stellate cell (HSC) activation. In the present study, we investigated the role of mitochondrial dynamics in HSC activation and the underlying mechanisms. We verified that mitochondrial fission was enhanced in human and mouse fibrotic livers and active HSCs. Moreover, increased mitochondrial fission driven by fis1 overexpression could promote HSC activation. Inhibiting mitochondrial fission using mitochondrial fission inhibitor-1 (Mdivi-1) could inhibit activation and induce apoptosis of active HSCs, indicating that increased mitochondrial fission is essential for HSC activation. Mdivi-1 treatment also induced apoptosis in active HSCs in vivo and thus ameliorated CCl₄-induced liver fibrosis. We also found that oxidative phosphorylation (OxPhos) was increased in active HSCs, and OxPhos inhibitors inhibited activation and induced apoptosis in active HSCs. Moreover, increasing mitochondrial fission upregulated OxPhos, while inhibiting mitochondrial fission downregulated OxPhos, suggesting that mitochondrial fission stimulates OxPhos during HSC activation. Next, we found that inhibition of oxidative stress using mitoquinone mesylate (mitoQ) and Tempol inhibited mitochondrial fission and OxPhos and induced apoptosis in active HSCs, suggesting that oxidative stress contributes to excessive mitochondrial fission during HSC activation. In conclusion, our study revealed that oxidative stress contributes to enhanced mitochondrial fission, which triggers OxPhos during HSC activation. Importantly, inhibiting mitochondrial fission has huge prospects for alleviating liver fibrosis by eliminating active HSCs.Subject terms: Endocrine system and metabolic diseases, Cell biology     

Tracking single hematopoietic stem cells in vivo using high-throughput sequencing in conjunction with viral genetic barcoding

Disentangling cellular heterogeneity is a challenge in many fields, particularly in the stem cell and cancer biology fields. Here we demonstrate how to combine viral genetic barcoding with high-throughput sequencing to track single cells in a heterogeneous population. We use this technique to track the in vivo differentiation of unitary hematopoietic stem cells (HSCs). The results are consistent with single-cell transplantation studies but require two orders of magnitude fewer mice. In addition to its high throughput, the high sensitivity of the technique allows for a direct examination of the clonality of sparse cell populations such as HSCs. We show how these capabilities offer a clonal perspective of the HSC differentiation process. In particular, our data suggest that HSCs do not equally contribute to blood cells after irradiation-mediated transplantation, and that two distinct HSC differentiation patterns co-exist in the same recipient mouse after irradiation. This technique can be applied to any virus-accessible cell type for both in vitro and in vivo processes.     

综述: 造血干细胞衰老的研究进展

成体干细胞衰老是组织器官老化的重要原因之一.越来越多的证据显示,免疫系统的衰老起始于造血干细胞(HSC)功能的下降,即造血干细胞的衰老直接影响免疫系统的功能.然而,有关HSC衰老的机理和分子机制仍旧不清楚.在这篇综述中,我们总结了造血干细胞衰老的表型,同时从细胞内在及外在两个方面探讨论了HSC衰老的分子机制.     

Bone-marrow haematopoietic-stem-cell niches

Adult stem cells hold many promises for future clinical applications and regenerative medicine. The haematopoietic stem cell (HSC) is the best-characterized somatic stem cell so far, but in vitro expansion has been unsuccessful, limiting the future therapeutic potential of these cells. Here we review recent progress in characterizing the composition of the HSC bone-marrow microenvironment, known as the HSC niche. During homeostasis, HSCs, and therefore putative bone-marrow HSC niches, are located near bone surfaces or are associated with the sinusoidal endothelium. The molecular crosstalk between HSCs and the cellular constituents of these niches is thought to control the balance between HSC self-renewal and differentiation, indicating that future successful expansion of HSCs for therapeutic use will require three-dimensional reconstruction of a stem-cell-niche unit.     

Mechanistic insights into the effects of SREBP1c on hepatic stellate cell and liver fibrosis

Sterol regulatory element‐binding protein 1c (SREBP1c) plays key roles in maintenance of hepatic stellate cell (HSC) quiescence. The present researches investigated the mechanisms underlying the effects of SREBP1c on HSCs and liver fibrogenesis by HSC‐targeted overexpression of the active SREBP1c using adenovirus in vitro and in vivo. Results demonstrated that SREBP1c exerted inhibitory effects on TAA‐induced liver fibrosis. SREBP1c down‐regulated TGFβ1 level in liver, reduced the receptors for TGFβ1 and PDGFβ, and interrupted the signalling pathways of Smad3 and Akt1/2/3 but not ERK1/2 in HSCs. SREBP1c also led to the decreases in the protein levels of the bromodomain‐containing chromatin‐modifying factor bromodomain protein 4, methionine adenosyltransferase 2B (MAT2B) and TIMP1 in HSCs. In vivo activated HSCs did not express cyclin D1 and cyclin E1 but SREBP1c down‐regulated both cyclins in vitro. SREBP1c elevated PPARγ and MMP1 protein levels in the model of liver fibrosis. The effect of SREBP1c on MAT2B expression was associated with its binding to MAT2B1 promoter. Taken together, the mechanisms underlying the effects of SREBP1c on HSC activation and liver fibrosis were involved in its influences on TGFβ1 level, the receptors for TGFβ1 and PDGFβ and their downstream signalling, and the molecules for epigenetic regulation of genes.